How to create smiling/crying animation?
You can do this with shape keys. First make the cube that had the smiling face. Then go to Properties > object data > shape keys and add a basis key. Now add another shape key.Go to the mesh and make the face to the frowning guy.  you can pan between the two keys slowly to make it form into the other. 



The higher you set the value, the more frowny your guy will appear.